1. Field of the Invention
The present invention relates to a new family of metastable intermolecular composites (“MICs”), and particularly to a family of metastable intermolecular composites utilizing energetic liquid oxidizers with nanoparticle fuels in a sol-gel polymer network.
2. Description of the Related Art
Metastable intermolecular composite (also called nanothermites or super-thermites) materials are a subclass of thermite materials in the nanometer length scale range. They are a pyrotechnic composition typically comprising an oxidizer and a reducing agent that undergoes an exothermic reaction when heated to a critical temperature. MICs are distinguished from conventional thermites in that the oxidizer and reducing agent, normally iron oxide and aluminum, are not a fine powder, but rather nanoparticles. As the mass transport mechanisms that slow down the burning rates of traditional thermites are not so important on the nano-scale, the reactions become kinetically controlled and much faster. Although they may be easily stimulated to become unstable, MICs exist in a state of pseudo-equilibrium that has a free energy higher than that of the true equilibrium state. Because of these and other advantages, MICs are offer improved performance over other energetic materials in areas such as sensitivity, stability, energy release and mechanical properties, and are becoming useful in applications in propellants, explosives, and pyrotechnics.
Conventional MIC formulations use solid oxidizer components being either metal oxides such as Fe2O3 CuO, MoO or KmnO4 with nano-sized fuel particles generally comprising one of or a mixture of aluminum, boron, beryllium, hafnium, lanthanum, lithium, magnesium, neodymium, tantalum, thorium, titanium, yttrium, zirconium, or other metals. Current MIC compositions employing metal oxidizers and metal fuels generate large amounts of heat, making them useful for applications for cutting metal and for the discharge of their main combustion product, hot metal fragments. However, traditionally MIC compositions have been relatively poor gas generators, making them a suboptimal candidate for propellant systems or gas generating control systems.
The rate of energy release in a MIC reaction is inversely proportional to the size of the MIC components. MICs comprising components on a nano-scale tend to be easier to ignite than traditional thermites, and indeed produce an explosion type reaction due to the large surface area and high amounts of heat generated by the reaction therein.
As a means for forming MICs, it is well recognized that the sol-gel process is an inexpensive, simple and efficient mechanism. The sol-gel process as it pertains to MIC formation involves reacting chemicals in a solvent to produce primary nanoparticles that are linked in a 3D solid network, the gaps in the 3D network filled in by the remaining solution. To isolate the MICs produced, the remaining solvent must be removed. The solvent may be removed through controlled evaporation or supercritical extraction, forming Xerogels in the former process and Aerogels in the latter. Regardless of the means of solvent removal, the finalized MIC product is left behind.
A first drawback to the formation of conventional MICs in this manner is with regard to the process of separating the solvent from the nanostructure. Unfortunately, this process has traditionally been detrimental to the preservation of the shape of the nanostructure framework. Indeed, either the conventional supercritical solvent extraction or the solvent extraction/evaporation steps result in a product that has either undergone complete 3D nanostructure collapse (in the case of Xerogels) or at the very least minor 3D nanostructure shrinkage (in the case of Aerogels). This damage to the 3D nanostructure eliminates the possibility of creating complex molded shapes due to the nanostructure pulling away from (or collapsing entirely within) any mold in which it was designed to fit. There is thus a need for creating a MIC with that does not undergo 3D collapse or shrinkage during preparation.
A second drawback to conventional MICs that utilize metal powders is that once initiated, their combustion may not be electrically controlled. Thus, in conventional MICs, burn rate and reactive power must be controlled indirectly through the control of particle size. Complete extinguishment is not possible. Rather, after initiation the conventional MIC reaction generates its own heat absent of any pressure effects, even if pressure drops to zero. Thus, in applications where conventional MICs may be used for igniters for ignition of solid propellants, they are limited to a one-time use. There is thus a need for an electrically controlled MIC to allow for multiple start-stop ignitions of solid, liquid, or hybrid propellant systems.
A third drawback to conventional MICs employing nano-sized metal is the high chance for accidental ignition by electrostatic discharge. That is, conventional MICs are spark sensitive. Currently, major considerations for successful weaponization of energetic materials include energy release rate, long-term storage stability, and sensitivity to unwanted initiation. Currently, conventional MICs are thus combined with carbon to reduce the chance of accidental electrostatic discharge. There is thus a need for a MIC that is not ignitable by accidental electrostatic discharge and that can eliminate the common step of combination with carbon.
A fourth drawback to conventional MICs involves their use in certain military, space and commercial applications wherein it is desirable that a propellant combust without a visible exhaust plume, such as for stealth purposes or because the exhaust particulates and smoke interfere with guidance control. Referred to as “smokeless” formulations, such formulations typically comprise no metal fuels or chlorine based oxidizers such as ammonium perchlorate. Conventional formulations utilize oxidizers referred to as nitramines and consist of 1,3,5-trinitro-1,3,5-triazacyclohexane (RDX) or 1,3,5,7-tetranitro-1,3,5,7 tetraazacyclooctane (HMX). More recently, newer higher nitrogen compounds Bis(aminotetrazolyl)tetrazine (BTATZ), dihydrazino-tetrazine (DHT) and Guanidinium azo tetrazolate (GUAZT) have been developed and proposed as additives that could be used with 5-amino tetrazole and potassium nitrate (KNO3) or potassium perchlorate (KClO4) to produce a reduced or smokeless MIC. To date, the cost of producing these materials is expensive and they have been found to be spark sensitive. There is thus a need for a MIC propellant combustible without a visible exhaust plume and that is both inexpensive to prepare and spark insensitive once prepared.
U.S. Pat. No. 5,734,124 to Bruenner, et al., entitled “Liquid Nitrate Oxidizer Compositions”, describes the formation of liquid nitrate eutectic compositions for solid solution or emulsion propellants wherein inorganic nitrate oxidizers are combined in eutectic compositions that place the oxidizers in liquid form at ambient temperatures, but that could used in the preparation of a wide variety of energetic formulations, notably solution and emulsion propellants made of ammonium nitrate, hydrazinium nitrate, hydroxylammonium nitrate and/or lithium nitrate, including eutectics. These propellants, which contain a metal fuel, a hydrocarbon polymer and the liquid oxidizer, form a gel structure that supports the metal fuel and may be used. No suggestion for an application to MICs is disclosed.
U.S. Patent Publication 2006/0053970 A1 to Dreizin and Schoenitz, entitled “Nano-composite energetic powders prepared by arrested reactive milling”, describes a method for producing an energetic metastable nano-composite material by arresting the milling process at a known duration before a spontaneous reaction is known to occur. The milled powder is recovered as a highly reactive nanostructured composite for subsequent use by controllably initiating destabilization thereof.
U.S. Patent Publication 2007/0095445 A1 to Gangopadhyay et al., entitled “Ordered nanoenergetic composites and synthesis method”, describes one such means for achieving the dispersion effect using a solvent and sonic waves (sonification). Here, the nano-sized fuel particles such as aluminum nanoparticles are placed in an alcohol solvent such as 2-propanol and are sonicated for a time sufficient to achieve homogenous dispersion and the removal of all of the molecular linker except the layer that is bound to the fuel or the oxidizer. A very high fuel surface area results, thereby increasing the explosive characteristics of the formulation. While this method has its advantages, it still relies on a solvent that must be extracted before the process is complete.